No - and you needn't ask again! (5)
I believe the answer is:
never
'needn't ask again' is the definition.
I can't tell whether this definition defines the answer.
'no and you' is the wordplay.
'no' becomes 'n' (common abbreviation).
'and' is a charade indicator (letters next to each other).
'you' becomes 'ever' (I can't explain this - if you can you should believe this answer much more).
'n'+'ever'='NEVER'
Can you help me to learn more?
(Other definitions for never that I've seen before include "Strong negative" , "' - - Say - - Again': Bond film" , "I refuse" , "Certainly not!" , "At no time whatever" .)